1. Field of the Invention
This invention relates to flash devices for cameras and, more particularly, to flash devices for cameras which vary the light distribution characteristic with variation of the focal length of the photographic optical system, or cameras using the same device.
2. Description of the Related Art
There have been many previous proposals for applying this kind of flash device to cameras. For example, a camera provided with a zoom strobe that varies the light distribution characteristic as the photographic lens moves for zooming purposes is known in Japanese Laid-Open patent application No. Sho 54-50324 and others. Further, a bifocal camera which varies the angle of illumination of flash light by varying the distance from the light emission portion of the strobe to the Fresnel lens in synchronism with changing over between the two focal lengths, too, is known in Japanese Laid-Open patent application No. Sho 59-64820 and others.
In the above-described conventional examples, however, a premise is set forth that the light emitting portion of the strobe is one unit of box shape. The introduction of a capability that the light distribution characteristic of the strobe is varied in response to zooming of the photographic lens, or changing over between the two focal lengths gives rise to drawbacks as described below.
(1) In order to be built in the compact camera, the discharge tube and reflector of the strobe must be decreased in size since the strobe has the form of the box like unit. Hence, it results that, if, as in the recently developed cameras, the focal length is made longer, the amount of emitted flash light tends to be insufficient, because such increase of the focal length leads to an increase in the F-number.
(2) For the same reason, the Fresnel lens in the front panel of the strobe unit cannot be designed to be large in size. Hence, the flash light could not sufficiently be condensed as compared with the add-on strobe for use with the single-lens reflex camera.
(3) Even in the type of apparatus in which the distance between the light emitter of the strobe and the Fresnel lens is made to vary as has been described before, because the availability of the space for this purpose is limited, it is difficult to set a sufficiently long stroke of motion.
(4) If the light emitter or the reflector is chosen to be made movable, the operating mechanism therefor gets complicated. As this is combined with the requirement of building the strobe unit into the interior of the camera, light shielding means also must be provided. A surplus space which is occupied by this means has to be created.